Coating, coating layer system, coated superalloy component

ABSTRACT

Coatings as may be used in a gas turbine are provided. A nickel based coating may include 15 to 40 wt % cobalt, 10 to 25 wt % chromium, 5 to 15 wt % aluminum, 0.05 to 1 wt % yttrium and/or at least one of elements from lanthanum series, 0.05 to 8 wt % ruthenium or iron, 0 to 1 wt % iridium, 0.05 to 5 wt % molybdenum, 0 to 3 wt % silicon, 0 to 5 wt % tantalum, 0 to 2 wt % hafnium, unavoidable impurities, and a balance of nickel. A cobalt based coating may include 15 to 40 wt % nickel, 15 to 28 wt % chromium, 5 to 15 wt % aluminum, 0.05 to 1 wt % yttrium and/or at least one of elements from lanthanum series, 0.05 to 5 wt % ruthenium and/or molybdenum, 0 to 2 wt % iridium, 0 to 3 wt % silicon, 0 to 5 wt % tantalum, hafnium, unavoidable impurities, and a balance of cobalt.

The invention relates to a coating, a coating layer system and a coatedsuperalloy component preferably to be used in a gas turbine's hot gaspath.

When further increasing engine efficiency, output power, availabilityand reliability in the current gas turbine development, this effort isoften limited by temperature capacity and lifetime of protectivecoatings for protection against hot corrosion and oxidation and bondingthermal barrier coating on the hot turbine components. The currentlyused coatings are so called MCrAlY coatings developed by major gasturbine manufactures. Most gas turbine manufactures use thereproprietary coatings since commercially available coatings are much lesseffective.

The term MCrAlY coatings is widely applied, wherein M means cobalt ornickel or a mixture of cobalt and nickel. These coatings may be used ascorrosion resistant overlays or as bond-coats for use with thermalbarrier coatings. Since in the first and second stage of a gas turbinemetal temperatures may exceed 850° C. two predominant corrosionmechanics have been identified to be accelerated. One is the hightemperature oxidation occurring at temperatures above 950° C. and theother mechanism is the so called type-I-hot-corrosion-occurring ataproximately 850° C. to 950° C.

During the last ten years the demand for coatings in gas turbinesincreased, which led to an increase in price of the coatings, whichcontain comparatively large amounts of rare earth or minor elementswhich's availability is increasingly tight.

Today's MCrAlY coatings rely very much on yttrium incorporation to haveso called pegging and scavenge effects to increase oxidation andcorrosion resistance of the coatings.

However, it has been recently found that the yttrium content in MCrAlYmay not be optimized. Nijdam T J, Sloof W G. (Acta Materia/ia2007;55:5980) reported that yttrium oxide inclusions in the protectivealuminum oxide scale on top of the MCrAlY provide fast oxygen diffusionroutes and therefore accelerate oxidation of the coating.

Further Smialek J L, Jayne Q T, Schaeffer J C and Murphy W H (Thin SolidFilms 1994; 253:285; Smialek J L; Metallurgical Transactions A. PhysicalMetallurgy and Material Science 1991;22A:739) state that a high sulfurcontent of bigger than 10 ppm (parts per million) existed in the currentMCrAlY layer shortening coating lifetime.

Typically ceramic coating(s) are used on a hot gas component of a gasturbine, for example turbine blades, combustors, transition ducts,sealing segments, and nozzle guide vanes, providing the components withthermal insulating or sealing functions. For the thermal insulating, theceramic coatings are called thermal barrier coatings (TBC) which arecharacterized by a low thermal conductivity and typically consist ofzirconia stabilized by yttria generally deposited by plasma spraying andelectron beam physical vapor deposition on the substrate. Often abond-coat is necessary to avoid exceeding residual stresses caused bydifferent thermal expansion of the substrate and the ceramic thermalbarrier coating(s). Most often the bond-coat is applied on the surfaceas a MCrAlY bond-coat.

It is one task of the invention to optimize the coating constitution, inorder to achieve a good thermal stability of coating phase structuresand a high mechanical durability. It is another task to optimize thecontent and combination of the rare earth and minor elements incoatings, in order to minimize growths of oxides on top of the coatingand inter-diffusion between coating and superalloy substrates which leadto a rapid consumption of the Al reservoir in coatings thereby coatinglifetime.

It is still another task of the invention to avoid the implementation ofsulfur in the coating system shortening the coating lifetime.

To overcome the technical limitations outlined above and as a reactionto the dramatic increase in price of rare earth elements a coatingaccording to claim 1, a coating layer system according to claim 7 and acoated superalloy component especially a blade, a vane, or a sealingsegment of a gas turbine hot gas path is proposed according to theinvention. The respectively dependent claims refer to preferredembodiments of the invention.

By introduction of other minor elements like ruthenium, iridium,molybdenum, silicon, hafnium, tantalum, and elements in lanthanum seriesthe necessary amount of yttrium is significantly reduced, whichefficiently minimize the negative effects of yttrium. The coating andcoating system according to the invention can be characterized by theterm MCrAlX alloy coating, wherein M stands for nickel or cobalt or bothof these elements and X is a combination of minor elements such asyttrium, ruthenium, iridium, silicon, hafnium, or tantalum and others.

The new coating alloy according to the invention performs in a veryefficient way because the introduction of ruthenium, molybdenum, or/andiridium reduces the diffusion rate of aluminum and forms a diffusionbarrier to minimize the inter-diffusion between the MCrAlX coating andthe substrate.

A preferred embodiment of the coating layer according to claim 1provides a reduction of the sulfur content to below 10 ppm whichincreases the coating lifetime.

Preferably the coating is applied in a thickness in the range of 30 to800 μm depending on the type of application and the application method.Preferred application methods are thermal spraying in air, thermalspraying in vacuum, thermal spraying in protected atmosphere, physicalvapor deposition, and plating on nickel or cobalt based superalloys.

The coating according to one of the claims 1 to 6 can be applied as asingle coating or as a bond-coat underlying an adherent ceramiccoating(s) compensating different thermal expansions between thesubstrate and the ceramic coating(s) on the one hand and improvingespecially the oxidation resistance of the superalloy component.

Summarizing this invention results in MCrAlX (as defined above) coatingswith a higher temperature capacity and longer lifetime compared toconventional MCrAlY coatings.

The following relates to preferred embodiments of the invention withreference to drawings illustrating the currently best mode of puttingthe invention into practice.

FIG. 1 shows a first embodiment of the invention, wherein the coating tothe invention is applied as a single layer to a substrate and

FIG. 2 shows a second embodiment of the invention, wherein the coatingaccording to claim 1 is an intermediate layer of a coating layer systemto be applied on a substrate.

FIG. 1 shows a substrate as covered with an adherent coating C of aconsistency outlined in claim 1. The coating C is applied on a substrateby way of thermal spraying in air or vacuum or protected atmosphere orby way of physical vapor deposition or by way of plating. The substrateis part of a superalloy gas turbine component, for example a gas turbinevane or a gas turbine blade or a combustor part.

FIG. 2 shows a substrate covered at least partially by a layer systemcomprising a coating as a lower layer directly provided on thesubstrate, which coating C is an intermediate layer provided as abond-coat for the adherent ceramic coating(s) TBC. The coating C has thecomposition outlined in claim 1. The substrate is a blade or a vane or acombustor part or a sealing segment of a gas turbine exposed to the hotgas.

The ceramic coating(s) consist of at least 70 wt % zirconium oxide andis stabilized by at least one of yttrium oxide, magnesium oxide, andoxides of elements in lanthanum series. The bond-coat compensatesdifferent thermal expansion between the ceramic coating(s) and thesuperalloy substrate.

1. A nickel based coating comprising: 15 to 40 weight percent of cobalt,10 to 23 weight percent of chromium,  5 to 15 weight percent ofaluminum, 0.05 to 1   weight percent of yttrium and/or at least one ofelements from lanthanum series, 0.05 to 5   weight percent of ruthenium,0 to 2 weight percent of iridium, 0.05 to 5   weight percent ofmolybdenum, 0 to 3 weight percent of silicon,  0 to 20 weight percent ofiron 0 to 5 weight percent of tantalum, 0 to 2 weight percent ofhafnium,

and unavoidable impurities, and a balance of nickel.
 2. The coatingaccording to claim 1, containing 0.1 to 0.6 weight percent of silicon.3. The according to claim 1, containing 0.3 to 0.7 weight percent oftantalum.
 4. The coating according to claim 1, containing 0.1 to 0.5weight percent of hafnium.
 5. A coating layer according to claim 1,wherein the content of sulfur is reduced to below 10 ppm (parts permillion).
 6. A coating layer according to claim 1, wherein the thicknessof the coating is between 30-800 μm.
 7. A coating layer systemcomprising a lower first layer on a substrate and an adherent uppersecond layer, wherein the first layer is a coating according to claim 1and the second layer is a ceramic coating or multiple ceramic coatings.8. The coating layer system according to claim 7, wherein each ofceramic coatings includes zirconium dioxide and a stabilizer.
 9. Thecoating layer system according to claim 7, wherein the stabilizer is atleast one of yttrium oxide, magnesium oxide, and oxides of elements inlanthanum series.
 10. A coated superalloy component comprising: asubstrate article formed of a superalloy and an adherent coatingaccording to claim
 1. 11. The coated superalloy component according toclaim 11, wherein the superalloy component is a blade, a vane, or asealing segment of a gas turbine's hot gas path.
 12. A coated superalloycomponent comprising: a substrate article formed of a superalloy and anadherent coating layer system according claim 8 covering at least aportion of the substrate article's surface.